The subject invention relates to a method and apparatus particularly suited for the nondestructive characterization of opaque layer structures on semiconductor samples.
There is a great need in the semiconductor industry for metrology equipment which can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710 and 5,074,669. The latter patents are incorporated herein by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample which spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way which alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface which alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused colinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes which are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
This system has the advantage that it is a non-contact, nondestructive -technique which can be used on product wafers during processing. Using lasers for the pump and probe beams allows for very tight focusing, in the micron range, to permit measurements with high spatial resolution, a critical requirement for semiconductor inspection. The prior system has been used extensively in the past to monitor levels of ion doping in samples since the modulated optical reflectivity is dependent on ion dopant levels in the sample. This dependence is relatively linear for the low to mid-dose regimes (1011 to 1014 ions/cm2). At higher dopant concentrations, the MOR signal tends to become non-monotonic and further information is needed to fully analyze the sample.
One approach for dealing with the problem of monitoring samples with high dopant concentrations is to measure the DC reflectivity of both the pump and probe beams in addition to the modulated optical reflectivity signal carried on the probe beam. Using the DC reflectivity data at two wavelengths, some ambiguities in the measurement can often be resolved.
The details of this approach are described in U.S. Pat. No. 5,074,669, cited above.
Semiconductor fabrication technology is increasing in complexity at a rapid pace. Various multilayer structures are being developed which makes testing more difficult. In addition, manufacturers are seeking to increase yields by fabricating chips on larger diameter wafers. As the diameter of the semiconductor wafers increases, the cost and value of each wafer increases. When using large, valuable and expensive wafers, it is no longer economically viable for manufacturers to rely on any forms of destructive testing methodologies. Therefore, there is a great need to provide equipment which can characterize complex structures with many unknowns or variables in a nondestructive manner.
Inspection problems also arise where metalized layers are deposited on semiconductors. If a typical metal layers is more than 100 angstroms thick, it will generally be opaque to more commonly used optical wavelengths. Therefore, equipment designed to monitor relatively transparent oxide layers cannot be effectively used to inspect metalized layers. Therefore, some new methodologies are required in order to inspect semiconductors with metalized layers. These layers can be formed from materials, such as aluminum, titanium, titanium nitride (TiN) and tungsten silicide (WSi).
In order to obtain sufficient information to characterize more complex samples, a system has been developed which substantially increases the amount of data that can be collected. The system of the subject invention includes an intensity modulated pump laser beam which is focused onto the sample in a manner to periodically excite the sample. A probe laser beam is focused onto the sample within the periodically heated area. A photodetector is provided for monitoring the reflected power of the probe beam and generating an output signal responsive thereto. The output signal is filtered and processed to provide a measure of the modulated optical reflectivity of the sample.
In accordance with the subject invention, the device further includes a steering means for adjusting the relative position of pump and probe beam spots on the sample surface. In the preferred embodiment, the steering means is used to move the beam spots from an overlapping, aligned position, to a separation of up to about 10 microns. Measurements can be taken as the separation of the beam spots is gradually changed or at discrete separation intervals.
This approach is particularly useful for monitoring the deposition of opaque, thin metal films. More specifically, the measurements taken at different spatial distances between the pump and probe beam spots can be used to help more accurately characterize the thermal diffusivity of the layer. This information can then be used by the processor to more accurately characterize the sample.
It should be noted that the concept of taking a measurement with a probe beam displaced from a pump beam can be found in the prior art. For example, in U.S. Pat. Nos. 4,521,118 and 4,522,510, both assigned to assignee herein and incorporated by reference, deformations at the sample surface, induced by periodic heating, are measured using a probe beam displaced from the pump beam. In the latter patents, periodic angular deviations of the probe beam are monitored. However, the latter patents do not teach or suggest that it would be desirable to take multiple measurements as the displacement between the two beams spots are varied.
Obtaining measurements from a probe beam displaced from a pump beam is also disclosed in U.S. Pat. No. 5,228,776, assigned to assignee herein and incorporated by reference. In this patent, an effort is made to align the pump and probe beams at the opposed ends of elongated conductive features within the sample. Further, the focal planes of the two beams are displaced vertically. In the principal embodiment of the U.S. Pat. No. 5,228,776 patent, the lateral spacing between the beams is selected and then fixed. There is no teaching in the U.S. Pat. No. 5,228,776 patent that it would be desirable to take multiple measurements as the displacement between the two beams is varied.
In the preferred embodiment of the subject invention, further. information can be obtained by varying the modulation frequency of the pump beam. While it has been known that obtaining information as a function of modulation frequency is useful, the subject invention expands upon the past teachings by increasing the modulation range. In particular, in the prior art, the modulation range was typically in the 100 KHz to 1 MHz range. Some experiments utilized modulation frequency as high as 10 MHz. In the subject device, it has been found useful to take measurements with modulation frequencies up to 100 MHz range. At these high frequencies, the thermal wavelengths are very short, enabling information to be obtained for thin metal layers on a sample, on the order of 100 angstroms.
In the preferred embodiment of the subject invention, further information can be obtained by varying the spot sizes of either the pump or probe beams. Varying the spot size of the pump beam will vary the propagation characteristics of the thermal waves. Varying the spot size of the probe beam will vary the sensitivity of the system with respect to the depth of detection. By taking measurements at different spot sizes, some depth profiling information can be recorded and used to characterize the sample.
In the preferred embodiment of the subject invention, still further information can be derived by obtaining independent reflectivity measurements at a plurality of wavelengths. More specifically, the subject apparatus can further include a polychromatic light source generating a second probe beam which is directed to the sample surface. The reflected beam is captured by a detector which is capable of measuring power as a function of wavelength. These added measurements can also be used to help better resolve ambiguities in the analysis and improve the characterization of the sample.
It is also possible to add additional measurement modules which measure either reflectivity or ellipsometric parameters as a function of angle of incidence. Further, the system can also be used to monitor the periodic angular deviations of the probe beam to derive additional information.
Further objects of the subject invention will become apparent from the following detailed description, taken in conjunction with the drawings, in which: